It has been known that a semiconductor device including a trench gate structure is configured to have higher channel density for making a larger current flowing through the channel. In this type of trench gate structure, there is a structure having one gate electrode arranged directly below another gate electrode (hereinafter, the gate electrode arranged at an upper side is called a first gate electrode; and the gate electrode arranged at a lower side is called a second gate electrode) and having connection between the second gate electrode and a source potential (for example, see Patent Document 1). With this configuration, a decrease in parasitic capacitance Cgd and relaxation in electric field at the bottom of a trench gate can be achieved.
In a case of including a double gate structure with two sides respectively having the first gate electrode and the second gate electrode, since a shield effect can be achieved because of the first gate electrode and the second gate electrode, the parasitic capacitance Cgd (feedback capacitance) generated between the first gate electrode and the drain can be decreased. Accordingly, the double gate structure can achieve high-speed switching as compare to a MOSFET with a single gate structure, which does not have a second gate electrode. In addition, in a case of having the second gate electrode, an influx of a high potential into a gate insulation film at the upper side can be inhibited. Therefore, the concentration of electric field at the bottom part of the trench gate at the upper side can be relaxed, and the improvement in a withstand voltage can be achieved.
However, when a drift layer is set to have higher concentration for aiming lower on-resistance in the MOSFET with a double gate structure, a larger electric field is applied to the bottom part of the double gate structure, and hence insulation breakdown may occur on the gate insulation film. In particular, in a case where a semiconductor includes a double gate structure made of SiC, an even larger current is applied to the bottom of the double gate structure as compare to a case of a semiconductor including a double gate structure made of Si; therefore, insulation breakdown may be further occurred on the gate insulation film. Since there is a trade-off between a decrease in the on-resistance and the strength of electric field applied to the gate insulation film, it is difficult to have lower on-resistance while having an improvement in a withstand voltage. Hence, it is desired to have the structure to lower on-resistance while having an improvement in a withstand voltage.